Electrode Material and Use Thereof for the Manufacture of an Inert Anode

Abstract

The invention relates to an electrode material, preferably an inert anode material comprising at least a metal core and a cermet material, characterized in that: said metal core contains at least one nickel (Ni) and iron (Fe) alloy, said cermet material comprises at least as percentages by weight: 45 to 80% of a nickel ferrite oxide phase (2) of composition Ni.sub.xFe.sub.yM.sub.zO.sub.4 with 0.60 x0.90; 1.90y2.40; 0.00z0.20 and M being a metal selected from aluminum (Al), cobalt (Co), chromium (Cr), manganese (Mn), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta) and hafnium (Hf) or being a combination of these metals, 15 to 45% of a metallic phase (1) comprising at least one alloy of nickel and copper.

Claims

1. Electrode material comprising at least a metal core and a cermet material, said metal core being at least covered by said cermet material and said cermet material forming an external layer of said electrode material which is designed to be in contact with an electrolysis bath, characterized in that: said metal core contains at least one nickel (Ni) and iron (Fe) alloy, the proportions by weight of Ni and Fe being as follows: 40%Ni85%, preferably 55%Ni80%, 15%Fe60%, preferably 20%Fe45%, said cermet material comprises at least as percentages by weight: 45 to 80% of a nickel ferrite oxide phase of composition Ni.sub.xFe.sub.yM.sub.zO.sub.4 with 0.60 x0.90; 1.90y2.40; 0.00z0.20 and M being a metal selected from aluminum (Al), cobalt (Co), chromium (Cr), manganese (Mn), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta) and hafnium (Hf) or being a combination of these metals, 15 to 45% of a metallic phase comprising at least one alloy of nickel and copper.

2. Electrode material according to claim 1, characterized in that the metal core of the electrode material further includes copper (Cu) in the following proportions by weight: 5%Cu40%.

3. Electrode material according to claim 2, characterized in that the proportions by weight of the metal core are: 40%Ni70%; 20%Fe45%; 7%Cu20%.

4. Electrode material according to claim 1, characterized in that the metal core of the electrode material further comprises at least one metal A, said metal A being selected from chromium (Cr), manganese (Mn), cobalt (Co) and molybdenum (Mo), with the proportion by weight of metal A in the metal core being as follows: 0.5%A30%.

5. Electrode material according to claim 4, characterized in that the proportions by weight of the metal core are: 40%Ni80%; 15%Fe40%; 0Cu20%; 0.5%A15%.

6. Electrode material according to claim 1, characterized in that the metal core further comprises at least one metal M selected from aluminum (Al), cobalt (Co), chromium (Cr), manganese (Mn), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta) and hafnium (Hf) or a combination of these metals, with the proportion by weight of metal M in the metal core being as follows: 0.5%M10%.

7. Electrode material according to claim 6, characterized in that the proportions by weight of the metal core are: 40%Ni80%; 15%Fe40%; 0Cu20%; 0.5A20%; 0.5%M5%.

8. Electrode material according to claim 1, characterized in that the metal core further comprises at least one rare earth element selected from yttrium (Y), cerium (Ce), lanthanum (La) and neodymium (Nd).

9. Electrode material according to claim 8, characterized in that the rare earth element counts for up to 5% of the mass of the metal core.

10. Electrode material according to claim 1, characterized in that nickel ferrite oxide phase of the cermet material has the composition Ni.sub.xFe.sub.yM.sub.zO.sub.4, with 0.70x0.85; 2.00y2.20; 0.00z0.10.

11. Electrode material according to claim 1, characterized in that the metal phase (1) comprising at least one NiCu alloy represents between 25% and 35% of the mass of the cermet material.

12. Electrode material according to claim 1, characterized in that in the alloy of nickel and copper that comprises at least said metal phase of the cermet material, the proportions by weight of Ni and Cu are as follows: 20%Ni90% and 10%Cu80%, and preferably 50%Ni90% and 10%Cu50%.

13. Electrode material according to claim 1, characterized in that the cermet material further comprises a monoxide phase of composition Ni.sub.xFe.sub.1-xO with 0.70x1.

14. Electrode material according to claim 13, characterized in that the percentage by weight of the monoxide phase in the cermet material is less than 10%.

15. Electrode material according to claim 13, characterized in that the monoxide phase is of composition Ni.sub.xFe.sub.1-xO with 0.75x0.85.

16. Electrode material according to claim 1, characterized in that the cermet material further comprises at least one rare earth oxide phase.

17. Electrode material according to claim 16, characterized in that the rare earth oxide is selected from Y.sub.2O.sub.3, CeO.sub.2, La.sub.2O.sub.3 and Nd.sub.2O.sub.3.

18. Electrode material according to claim 16, characterized in that the rare earth oxide phase is at most 5% of the weight of the cermet material.

19. Electrode material according to claim 1, characterized in that the metallic phase of the cermet material further comprises gold (Au) and/or silver (Ag), the percentage by weight of these metals Au and/or Ag not exceeding 5% of the weight of said metallic phase of the cermet material.

20. Electrode material according to claim 1, characterized in that a thickness of the cermet material is greater than or equal to 1 mm.

21. Electrode material according to claim 20, characterized in that the thickness of the cermet material is between 2 and 8 mm.

22. Electrode material according to claim 1, characterized in that the electrode material further comprises at least one intermediate layer arranged between the metal core and the cermet material, said intermediate layer containing at least nickel and being predominantly metallic.

23. Electrode material according to claim 22, characterized in that a total thickness of the cermet material and the intermediate layer is greater than or equal to 1 mm, said cermet material having a thickness of at least 0.5 mm.

24. Electrode material according to claim 22, characterized in that the intermediate layer is a layer of nickel having a thickness of between 200 and 300 microns.

25. Electrode material according to claim 22, characterized in that the intermediate layer is a layer of cermet material comprising more than 50% by volume of a metallic phase containing at least nickel.

26. Method of manufacturing an electrode material according to claim 1, when the electrode material further comprises at least one intermediate layer, characterized in that the manufacturing process comprises at least the following steps: preparing the metal core of said electrode material according to a method selected from casting, molding, rolling, hot working such as rolling, extrusion, or powder metallurgy; and optionally depositing at least one intermediate layer is on the metal core; depositing the cermet material on the metal core, or as appropriate on the last deposited intermediate layer, said deposition being carried out by a method selected from the methods of spraying or powder metallurgy.

27. Inert anode made from an electrode material according to claim 1.

28. Electrolysis cell comprising at least one inert anode according to claim 27.

29. A method comprising producing aluminum by electrolysis in an electrolysis cell according to claim 28.

Description

DESCRIPTION OF THE ONLY FIGURE

[0214] FIG. 1 is a photograph of an observation by backscattered electron SEM of a portion of the inert anode of example 3 according to the invention.

EXPERIMENTAL PART

[0215] Examples of electrode materials, firstly for purposes of comparison with respect to the invention and secondly according to the invention and their use as inert anodes during electrolysis are described below.

[0216] For all the experiments described below, the electrolysis conditions were as follows: a cryolite bath with a cryolitic ratio of 2, the cryolitic ratio being the ratio of mole percentages of NaF over AlF.sub.3, with Al.sub.2O.sub.3 to saturation point and 5% of CaF.sub.2 at a temperature of 960 C. and with a current of 0.8 A/cm.sup.2.

ICOMPARATIVE EXAMPLES

Example A

1.SUP.st .Comparative Example of an Anode

[0217] An anode in the form of a cylinder 20 mm in diameter and formed from a cermet material composed of a nickel ferrite oxide phase Ni.sub.o.9Fe.sub.2.3O.sub.4, in other words pure nickel ferrite, was manufactured.

[0218] This anode was subjected to electrolysis for 96 hours under the conditions described above.

[0219] At the end of the electrolysis, it was found that the anode had been deformed and had a general shape similar to that of a diabolo indicating that the working area of the anode had been the interface between the cryolite bath and the gas atmosphere.

[0220] This deformation of the anode can be explained by the fact that pure nickel ferrite oxide oxidizes rapidly and becomes less conductive than the cryolite bath.

[0221] So from this example A it is noted that pure nickel ferrite oxide is not a suitable material for the manufacture of an inert anode to be used for igneous electrolysis in the production of aluminum.

Example B

2.SUP.nd .Comparative Example of an Anode

[0222] An anode formed only from a cermet material containing, as percentages by weight: [0223] 67% of a nickel ferrite oxide phase, of composition: Ni.sub.0.77Fe.sub.2,19Al.sub.0.04O.sub.4; [0224] 2% of an NiO phase; [0225] 2% of Y.sub.2O.sub.3; [0226] 29% of a metal phase of a NiCu alloy comprising, as percentages by weight, 85% nickel and 15% copper.

[0227] was tested for 360 hours in electrolysis conditions as detailed above.

[0228] It should be noted that in this example B, the cermet material of this anode corresponds to a cermet material as described above, namely a cermet material that the electrode material according to the invention may contain.

[0229] After 360 hours, scanning electron microscopy revealed that the anode then had a highly porous microstructure.

[0230] In addition, at the surface of the anode, the metallic phase had completely disappeared over a thickness of 2.9 mm.

[0231] Only the nickel ferrite oxide phases and nickel oxide were present at the surface of the anode and were distributed in the form of a superposition of layers parallel to each other extending toward the surface of the anode.

[0232] The oxide layer of nickel ferrite nearest the surface of the anode had the following composition: NiFe.sub.2O.sub.4, namely a non-conductive composition.

[0233] After a certain time, the anode of this example B was no longer sufficiently conductive. The anode did not have any means to regenerate iron from the nickel ferrite oxide phase which disappeared gradually as the electrolysis proceeded.

[0234] This example B therefore shows that an anode which contains only a cermet material as described above is not satisfactory for use in electrolysis for the production of aluminum.

IIEXAMPLES ACCORDING TO THE INVENTION

[0235] For all electrode materials in examples 1-5 of the invention which follow, the manufacturing process was as follows:

[0236] The manufacture of the cermet material that the electrode material according to the invention contains was conducted as follows: [0237] 1) First a nickel ferrite oxide of composition Ni.sub.xFe.sub.3-xO.sub.4 with a slight excess of NiO was prepared by performing the following steps: [0238] a mixture was prepared which included 65.8% of Fe.sub.2O.sub.3 powder and 34.2% of NiO powder; [0239] then the mixture so obtained was subjected to heat treatment for 5 hours in air at 1150 C. so as to obtain nickel ferrite oxide powder. [0240] 2) Next, in order to obtain a cermet material in powder form, this nickel ferrite oxide powder as obtained in step 1) above was mixed with powders of nickel, copper, alumina and optionally yttria (with compositions detailed in the examples below), and 1 to 5% of organic binder was added to the mixture.

[0241] Then, to manufacture the inert anode, the following steps were performed: [0242] The cermet material powder so obtained was pressed onto a metal core composition (said metal core composition is described in each of the inventive examples below); [0243] This was sintered (by heat treatment) in a controlled atmosphere to a maximum temperature of between 1100 C. and about 1350 C.

[0244] After sintering, the inert cermet anode material contained the following phases: [0245] a nickel ferrite oxide phase of composition Ni.sub.0.75Fe.sub.2,20Al.sub.0.05O.sub.4, [0246] a monoxide phase of composition Ni.sub.0.85Fe.sub.0.15O, [0247] a metallic phase of NiCu alloy comprising a mixture, in weight percentages, of 85% Ni and 15% Cu, [0248] where appropriate, one to two yttria phases (depending on the compositionssee in examples according to the invention below).

Example 1

1.SUP.st .Example of an Anode According to the Invention

[0249] An inert anode was manufactured from an electrode material according to the present invention, which contained: [0250] a metal core containing a mixture of nickel, iron and molybdenum in the following proportions by weight: 80% nickel, 15% iron and 5% molybdenum; [0251] a cermet material with a thickness of 8 mm, said cermet material containing, as percentages by weight: [0252] 67% of a nickel ferrite oxide phase of composition Ni.sub.0.75Fe.sub.2,20Al.sub.0.05O.sub.4; [0253] 1.5% of a monoxide phase of composition Ni.sub.0.85Fe.sub.0.15O; [0254] 31% of a metal phase of a NiCu alloy comprising, as percentages by weight, 85% nickel and 15% copper. [0255] and 0.5% of a Y.sub.2O.sub.3 phase.

[0256] It should be noted that this cermet material corresponds to a cermet material of the same type as that of the anode in comparative example B, comparable from the point of view of its physicochemical properties.

[0257] After 96 hours of electrolysis in the conditions as detailed above, no wear damage on the anode of this example 1 was detected.

[0258] A layer of pure nickel ferrite oxide is to be found on the surface of the anode.

[0259] Under this layer, the three phases of the cermet material (i.e. a nickel ferrite oxide phase, a monoxide phase and a metallic phase) are still present.

[0260] The results from example 1 are different from the results obtained with the anode of example B (an anode which, for the record, included only a cermet material of the same type).

[0261] So this example 1 demonstrates the effectiveness of the electrode material according to the invention. From the electrode material according to the invention an inert anode is obtained that is perfectly suitable for use in igneous electrolysis for the production of aluminum.

[0262] In addition, in view of the different results obtained between example B and example 1, this Example 1 demonstrates the advantage that the electrode material includes not only a cermet material but also a metal core.

Example 2

2.SUP.nd .Example of an Anode According to the Invention

[0263] An inert anode with outside diameter 34.4 mm was manufactured from an electrode material according to the present invention. Said electrode material contained: [0264] a metal core containing a mixture of nickel, iron and copper in the following proportions by weight: 65% nickel, 25% iron and 10% copper; [0265] a cermet material covering the side wall of the metal core by a thickness of 7 mm and the bottom wall of the metal core by a thickness of 16 mm. [0266] Said cermet material contained as percentages by weight: [0267] 66% an nickel ferrite oxide phase of composition: Ni.sub.0.75Fe.sub.2.20Al.sub.0.05O.sub.4; [0268] 1.5% a monoxide phase of composition Ni.sub.0.85Fe.sub.0.15O; [0269] 31% a metal phase of a NiCu alloy comprising, as percentages by weight, 85% nickel and 15% copper. [0270] and 1.5% of a Y.sub.2O.sub.3 phase.

[0271] Similarly to example 1, it should be noted that this cermet material corresponds to a cermet material of the same type as that of the anode in comparative example B.

[0272] The anode of example 2 was subjected to electrolysis for a period of 96 hours in the electrolysis conditions as described above.

[0273] The anode of example 2 showed a steady potential during the 96 hours of testing.

[0274] Furthermore, after 96 hours of electrolysis, it was found that the anode was intact. It was also observed that the three phases of the cermet material (i.e. a nickel ferrite oxide phase, a monoxide phase and a metallic phase) were still present.

[0275] The results from example 2 are different from the results obtained with the anode of example B (an anode which, for the record, included only a cermet material of the same type).

[0276] So this example 2 demonstrates the effectiveness of the electrode material according to the invention. From the electrode material according to the invention an inert anode is obtained that is perfectly suitable for use in igneous electrolysis for the production of aluminum.

[0277] In addition, in view of the different results obtained between example B and example 2, this Example 2 demonstrates the advantage that the electrode material includes not only a cermet material but also a metal core.

[0278] In other words, this example 2 demonstrates the beneficial interactions between the metal core and the cermet material in the electrode material according to the invention.

Example 3

3.SUP.rd .Example of an Anode According to the Invention

[0279] A 3.sup.rd anode according to the present invention was manufactured to the same composition as that of example 2 and therefore very similar to it.

[0280] Specifically, the anode of example 3 differed from that of example 2 only by some differences in the dimensions, namely: [0281] the thickness of the cermet material on the side wall of the metal core was 8 mm; [0282] the thickness of the cermet material on the bottom wall of the metal core was 1.2 mm; [0283] the outer diameter of the anode was 34.4 mm.

[0284] After 96 hours of electrolysis, the cermet material was still present on the anode. Moreover, there was no evidence of wear damage to the anode.

[0285] The three initial phases of the cermet material (i.e. the nickel ferrite oxide phases, monoxide and metal) were still present.

[0286] A thin layer of nickel ferrite oxide of composition Ni.sub.0.9Fe.sub.2.1O.sub.4 very close to the composition of pure nickel ferrite oxide, was formed on the surface of the anode, and the cermet material under this thin layer was composed as follows: [0287] nickel ferrite oxide phase of composition Ni.sub.0.82Fe.sub.2,12Al.sub.0.05O.sub.4, [0288] monoxide phase of composition Ni.sub.0.8Fe.sub.0.2O.

[0289] The metallic phase of the cermet material still contained nickel and copper.

[0290] As regards the inert anode of this example 3 according to the invention, FIG. 1 is a photograph of an observation by backscattered electron SEM of a portion of this inert anode taken after 96 hours of electrolysis and after having been resin-coated, cut and polished. More specifically, the part photographed is the cermet material that this anode includes.

[0291] The characteristics of this observation by SEM were: micrograph showing an area of 460 microns by 1.2 millimeters, at the level of the bottom wall of the anode.

[0292] The photograph in FIG. 1 shows the different phases of the cermet material present, which are: [0293] phase 1, metallic, of NiCu alloy (white dots), [0294] phase 2 of nickel ferrite oxide Ni.sub.xFe.sub.yAl.sub.zO.sub.4 (dark gray dots), [0295] phase 3, monoxide Ni.sub.xFe.sub.1-xO (light gray dots), [0296] and porosities 4 (black dots).

[0297] In addition, in FIG. 1 the various limitations are indicated: [0298] interface: The interface between the cermet material and the metal core of the inert anode; [0299] reduced zone: The area of material located between the cermet material and the metal core of the inert anode; [0300] buffer zone: The chemical buffer zone of the cermet material, i.e. the core of the cermet material; [0301] nickel ferrite oxide layer: The outer side of the cermet material, namely the layer of the cermet material which was in contact with the cryolite bath during electrolysis.

[0302] It can be seen that the part of the cermet material that was in contact with the cryolite bath during electrolysis has been transformed into a fine nickel ferrite oxide layer of forty microns or so dense (dark gray color of the dots in FIG. 1).

[0303] The part of the cermet material in contact with the metal core of the inert anode has a significant proportion of metal (white dots in FIG. 1), demonstrating the reduction of the cermet material by the metal core during electrolysis.

[0304] Finally, given the importance of the buffer zone in FIG. 1, it is noted that the majority of the cermet material was only slightly transformed: despite the presence of pores, the three phases present are still visible and therefore still present after 96 hours of electrolysis.

Example 4

4.SUP.rd .Example of an Anode According to the Invention

[0305] An inert anode with outside diameter 24.2 mm was manufactured from an electrode material according to the invention. Said electrode material contained: [0306] a metal core containing a mixture of nickel, iron and copper in the following proportions by weight: 65% nickel, 25% iron and 10% copper; [0307] a cermet material with a thickness of 2 mm. [0308] Said cermet material contained as percentages by weight: [0309] 67% of a nickel ferrite oxide phase, of composition: Ni.sub.0,75Fe.sub.2,20Al.sub.0,05O.sub.4; [0310] 1% of a monoxide phase of composition Ni.sub.0.85Fe.sub.0.15O; [0311] 32% of a metal phase of a NiCu alloy comprising, as percentages by weight, 85% nickel and 15% copper.

[0312] After 213 hours of electrolysis under the conditions as described above, the anode according to Example 4 was intact.

Example 5

5.SUP.th .Example of an Anode According to the Invention

[0313] A 5.sup.th anode according to the present invention was manufactured to the same composition as that of example 4 and therefore very similar to it.

[0314] Specifically, the anode of example 5 differed from that of example 4 only by some differences in the dimensions, namely: [0315] the outer diameter of the anode was 34 mm. [0316] the thickness of the cermet material was 8 mm.

[0317] After 404 hours of electrolysis under conditions which have been detailed above, it can be seen that the anode has undergone only minor wear damage.

[0318] In addition, the cermet material is still present on the anode and contains three phases (i.e., nickel ferrite oxide phase, a monoxide phase and a metal phase)

[0319] The results from example 5 are different from the results obtained with the anode of example B (an anode which, for the record, included only a cermet material of the same type).

[0320] So this example 5 demonstrates the effectiveness of the electrode material according to the invention. From the electrode material according to the invention an inert anode is obtained that is perfectly suitable for use in igneous electrolysis for the production of aluminum.

[0321] In addition, in view of the different results obtained between example B and example 5, this Example 5 demonstrates the advantage that the electrode material includes not only a cermet material but also a metal core.